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Field Guides & Natural History  Evolutionary Biology  Evolution

Across the Bridge Understanding the Origin of the Vertebrates

Popular Science
By: Henry Gee(Author)
312 pages, 19 b/w photos, 12 b/w illustrations
Across the Bridge is a timely overview of the oft-neglected topic of the evolution of vertebrates and our closest invertebrate cousins.
Across the Bridge
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  • Across the Bridge ISBN: 9780226403052 Paperback Jul 2018 In stock
  • Across the Bridge ISBN: 9780226402864 Hardback no dustjacket Jul 2018 Out of Print #240392
Selected version: £23.99
About this book Contents Customer reviews Biography Related titles

About this book

Our understanding of vertebrate origins and the backbone of human history evolves with each new fossil find and DNA map. Many species have now had their genomes sequenced, and molecular techniques allow genetic inspection of even non-model organisms. But as longtime Nature editor Henry Gee argues in Across the Bridge, despite these giant strides and our deepening understanding of how vertebrates fit into the tree of life, the morphological chasm between vertebrates and invertebrates remains vast and enigmatic.

As Gee shows, even as scientific advances have falsified a variety of theories linking these groups, the extant relatives of vertebrates are too few for effective genetic analysis. Moreover, the more we learn about the species that do remain – from sea-squirts to starfish – the clearer it becomes that they are too far evolved along their own courses to be of much use in reconstructing what the latest invertebrate ancestors of vertebrates looked like. Fossils present yet further problems of interpretation. Tracing both the fast-changing science that has helped illuminate the intricacies of vertebrate evolution as well as the limits of that science, Across the Bridge helps us to see how far the field has come in crossing the invertebrate-to-vertebrate divide – and how far we still have to go.



Chapter One: What Is A Vertebrate?
      1.1 Vertebrates in Context
      1.2 What Makes a Vertebrate?
      1.3 Breaking Branches
      1.4 Summary
Chapter Two: Shaking the Tree
      2.1 Embranchements and Transformation
      2.2 Evolution and Ancestors
      2.3 Summary
Chapter Three: Embryology and Phylogeny
      3.1 From Embryos to Desperation
      3.2 Genes and Phylogeny
      3.3 Summary
Chapter Four: Hox and Homology
      4.1 A Brief History of Homeosis
      4.2 The Geoffroy Inversion
      4.3 The Phylotypic Stage
      4.4 The Meaning of Homology
      4.5 Summary
Chapter Five: What Is A Deuterostome?
Chapter Six: Echinoderms
Chapter Seven: Hemichordates
Chapter Eight: Amphioxus
Chapter Nine: Tunicates
Chapter Ten: Vertebrates
Chapter Eleven: Some Non-deuterostomes
Chapter Twelve: Vertebrates from the Outside, In
      12.1 Introduction
      12.2 The Organizer
      12.3 The Notochord
      12.4 Somitogenesis
      12.5 Segmentation and the Head Problem
      12.6 The Nervous System
      12.7 Neural Crest and Cranial Placodes
      12.8 The Skeleton
      12.9 Summary
Chapter Thirteen: How Many Sides Has A Chicken?
      13.1 Introduction
      13.2 The Enteric Nervous System
      13.3 The Blood and the Heart
      13.4 The Urogenital System
      13.5 The Gut and Its Appendages
      13.6 Immunity
      13.7 The Pituitary Gland
      13.8 Summary
Chapter Fourteen: Some Fossil Forms
      14.1 Fossils in an Evolutionary Context
      14.2 Meiofaunal Beginnings
      14.3 Cambroernids
      14.4 Vetulicystids
      14.5 Vetulicolians
      14.6 Yunnanozoans
      14.7 Pikaia
      14.8 Cathaymyrus
      14.9 The Earliest Fossil Vertebrates
      14.10 Conodonts
      14.11 Ostracoderms and Placoderms
      14.12 Summary
Chapter Fifteen: Breaking Branches, Building Bridges
      15.1 Defining the Deuterostomes
      15.2 Ambulacraria
      15.3 Echinoderms
      15.4 Hemichordates
      15.5 Chordates
      15.6 Amphioxus
      15.7 The Common Ancestry of Tunicates and Vertebrates
      15.8 Tunicates
      15.9 Vertebrates
      15.10 Cyclostomes
      15.11 Gnathostomes
      15.12 The Evolution of the Face
      15.13 Crossing the Bridge
      15.14 Conclusions

Customer Reviews (1)

  • Timely overview of this neglected topic
    By Leon (NHBS Catalogue Editor) 13 Dec 2018 Written for Paperback

    When you think of an animal, you will most likely think of a vertebrate. Since we are animals with a backbone ourselves, it is not strange that that which is closest to us comes to mind first. But when and how did vertebrates evolve? To answer that question, Nature editor Henry Gee takes a good hard look at invertebrates, convincing the reader that they are not all equal. More than 20 years ago, Gee wrote Before the Backbone, which took a look at historical explanations for the origins of vertebrates. Which group of invertebrates is closest to us remains a topic of active research and Across the Bridge brings readers up to date with our current thinking.

    Gee uses the first few chapters to bring the reader up to speed on what a vertebrate actually is, how evolution works and how scientists make hypotheses about relatedness, how certain genes (Hox genes and others) influence gross morphology, and how embryos develop. That last topic is a fluke of history, for scientists in the 18th and 19th century used to think that the way an embryo develops mirrors evolutionary progress from simple to complex organisms. It gave rise to that hackneyed expression that “ontogeny recapitulates phylogeny”, an expression inseparable from the name of German naturalist Ernst Haeckel. His drawings of developing embryos are iconic, contested, but ultimately not entirely wrong (See Haeckel's Embryos for the definitive history of those, and my (long-winded) review of that book here).

    The topic of embryology brings with it a lot of terminology that you may or may not remember from your zoology classes: the names of tissue layers (ecto-, endo-, and mesoderm), other structures (blastopore, coeloms, notochord), and processes (gastrulation, neurulation, enterocoely, and schizocoely). Luckily, Gee includes plenty of helpful schematic drawings.

    Vertebrates, together with a number of invertebrate groups, belong to the deuterostomes, a distinction that is based on what an embryo develops first, the mouth or the anus. Adding more terminology, a substantial part of the book first reviews the morphology and development of those invertebrate groups closest to the vertebrates, including the echinoderms (think sea stars), hemichordates, tunicates, and Amphioxus. He then cuts the cake the other way and looks at typical vertebrate organs and features and asks what equivalents our closest invertebrate cousins have where, say, the heart, immune system, head, or brain is concerned.

    All this terminology notwithstanding, Gee does a good job of keeping his text as readable as can be reasonably expected. He inserts a touch of humour here and there. He makes observations in his footnotes that delighted both my inner five-year-old (comb jellies have several anuses!) and my inner 38-year-old (would our thinking about evolution have such a close link with development if early naturalists had not been so interested in embryology?). But most important and helpful: most chapters end with a summary. Who said that summaries should only be something found in student textbooks?

    The other refreshing part of this book is how it corrects many potential misunderstandings about how evolution proceeds. There is no linear progression from simple to complex organisms. Some lineages evolve faster than others. Invertebrates today are not some frozen primitive snapshot of life hundreds of millions of years ago but have kept on evolving since they split off from the last common ancestor between us and them (you are more likely to have heard this in the context of the evolution of humans and other primates ). And what can really throw a spanner in the works: organisms can lose complex traits again.

    Technological advances over the last few decades have allowed detailed studies of genes and whole genomes, and have birthed the study of the evolutionary relatedness of genes between organisms – a discipline known as evolutionary developmental biology, or evo-devo for short (see Endless Forms Most Beautiful for an accessible introduction, and From Embryology to Evo-Devo for a deeper study). These advances, and the aforementioned spanner in the works (the loss of complex traits), have brought about at least one revolutionary change in our thinking which Gee relates here with gusto. Of all the invertebrate cousins described, tunicates (sessile or free-floating marine filter feeders such as sea squirts that look nothing like us) are more closely related to us than Amphioxus (segmented, vaguely fish-like invertebrates also known as lancelets that do resemble us). Closer inspection shows it makes sense, but also shows the remarkable evolutionary history of tunicates that have lost many traits. All this serves as a powerful reminder that evolution is far from a neat and tidy story of progression.

    Gee closes out with a short look at fossils, which seems a bit of a non-starter for creatures that lack hard structures such as bones and teeth. The deuterostomes diversified in the Cambrian, between 541 to 485 million years ago, and there are a few localities on the planet that have yielded exceptional invertebrate fossils of the very distant ancestors of vertebrates and all the other deuterostome groups mentioned here so far. One of these is the Burgess Shale in British Columbia, made famous by Gould’s book Wonderful Life, but the Chengjiang biota of southern China is equally astounding, as documented in The Cambrian Fossils of Chengjiang, China.

    If your interest in evolution goes well beyond extinct mammals and reptiles, you can handle a bit of zoological jargon, and Chordate Origins and Evolution strikes you as too technical, then Across the Bridge is a wonderfully readable overview of how we vertebrates relate to our closest invertebrate cousins. Although much remains unclear and there is plenty of scope for further research, this book is a timely overview of the current state of knowledge on this neglected topic.
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Henry Gee is a senior editor at Nature and the author of such books as Jacob's Ladder, In Search of Deep Time, The Science of Middle-Earth, and, most recently, The Accidental Species: Misunderstandings of Human Evolution, the last published by the University of Chicago Press. He lives in Cromer, Norfolk, England, with his family and numerous pets.

Popular Science
By: Henry Gee(Author)
312 pages, 19 b/w photos, 12 b/w illustrations
Across the Bridge is a timely overview of the oft-neglected topic of the evolution of vertebrates and our closest invertebrate cousins.
Media reviews

"An excellent addition, complementing Gee's earlier book Before the Backbone, which provided a historical perspective on ideas surrounding vertebrate origins. Gee addresses an important topic for biologists and zoologists about vertebrates' place in the 'grand scheme.' We are familiar with vertebrates, or think that we are. However, Gee shows beautifully, as a group we are just as strange in many ways as other groups appear to us. Across the Bridge takes on a very esoteric subject and is genuinely witty and charming. The book really is magnificent."
– Neil J. Gostling, University of Southampton

"In Across the Bridge, Gee writes a beautiful ode to some of the least appreciated animals, who reveal our own evolutionary origins: the deuterostomes. Combining a sense for detail and prosaic ease, Gee guides the reader joyfully through deuterostomes – weaving disparate elements of embryology, paleontology, and morphology into an unprecedented and accessible narrative. This book not only gives a state of affairs – being an excellent primer for anyone interested in early animal evolution – it also proposes novel, compelling, and challenging hypotheses for researchers to test for decades to come. As senior editor of Nature, Gee has had a first-row seat to the revelations made across the disciplines of evolutionary biology for almost thirty years. Here, he has applied his polyhistoric expertise in this field to propose a vision for future interdisciplinary research, as well as created what will surely become a classic textbook for future generations of students."
– Jakob Vinther, University of Bristol

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